Wednesday, November 28, 2012

JAMA: Waning Pertussis Vaccine Effectiveness Over Time

Pertussis, or `whooping cough’ was once a major childhood disease in the United States, with more than a million cases reported between 1940 and 1945, resulting in thousands of deaths.

All that began to change with the introduction of a whole cell Pertussis vaccine in the 1940s. By 1960, the number of Pertussis cases in the United States had dropped to about 15,000 cases a year – a 95% reduction over 20 years.

Rates dropped another 2/3rds over the next 10 years (see chart above).

The vaccine, while highly effective, was blamed for a variety of adverse reactions, prompting a move away from whole cell pertussis vaccines to safer – but less broadly protective - acellular vaccines in the 1990s

The following chart illustrates the rise in pertussis cases, which began in earnest in the early 1990s. The most recent years (2010-2011) are not shown, but they were among the most active in decades.

While still far below the rates seen in the 1940s, the recent rise in Pertussis cases has raised a good deal of concern these past couple of years. A few blogs on it include:

Today we can add another study, this time appearing in JAMA, that finds that the protective effect of the Pertussis vaccine begins to wane after the fifth dose (normally given at age 5) is received, leaving 7 to 10 year olds at greater risk of infection.

Conclusion Among children in 15 California counties, children with pertussis, compared with controls, had lower odds of having received the 5-dose DTaP series; as time since last DTaP dose increased, the odds increased, which is consistent with a progressive decrease in estimated vaccine effectiveness each year after the final dose of pertussis vaccine.

Pertussis remains a poorly controlled vaccine-preventable disease in the United States, despite a well-established childhood vaccination program and high coverage. Although infants have substantially higher rates of pertussis compared with other age groups, data from the National Notifiable Diseases Surveillance System reflect a recent increase in the number of reported pertussis cases among children aged 7 to 10 years. In 2010, this age group had the second highest incidence of pertussis in the United States.

The odds of a child developing pertussis increase as the interval since he or she received the fifth and final dose of the DTaP vaccine increases, according to a report published in the November 28, 2012 issue of JAMA.

This pattern indicates a progressive waning of vaccine effectiveness every year after completion of the vaccine series, which would explain the recently noted surge in pertussis cases among 7- to 10-year-olds in at least 34 states, said Lara K. Misegades, Ph.D., of the Meningitis and Vaccine Preventable Disease Branch of the Centers for Disease Control and Prevention and her associates.

As more confirmatory data comes in and is analyzed, the question then becomes what to do about this decline in protection for those aged 7 to 10 years of age. The authors, in their conclusion, grant that their are no easy answers.

The increasing incidence of pertussis, changing epidemiology, and demonstrated decline in the estimated DTaP VE over time have raised concerns about the current US pertussis vaccine program and may prompt consideration of alternative schedules.

Options include delaying administration of the fifth DTaP dose or administering the Tdap booster at earlier than 11 years of age. However, a recommendation to delay the fifth DTaP dose until 6 years of age or later may unintentionally increase the burden of disease between the fourth and fifth doses of the childhood series, and implementation would likely be programmatically challenging because many states' school entry immunization requirements for pertussis are built around the current DTaP schedule.

Alternatively, shifting the Tdap booster to 10 years of age or earlier may have the unwanted effect of reducing coverage, as there is no established routine health care visit for children before the adolescent vaccine platform visit at 11 to 12 years of age.32

Given the options for adjustments to the pertussis vaccine schedule, these issues will require careful and ongoing review of the epidemiology and vaccine program nationwide. Ultimately, improved control of pertussis may require a vaccine that provides longer duration of protection or differently affects transmission in the community.